Underground Installations: Armored clad cable Type AC

By Michael Johnston | Feb 15, 2024
dentist chair in blue background
I was recently asked by a contractor about a violation they received from the electrical inspector. 




I was recently asked by a contractor about a violation they received from the electrical inspector. The nonconformity was related to installing Type AC cable through a PVC conduit in the floor of a dental office.

The branch circuit was being installed to supply power to dental chair equipment. The inspector flagged two issues. The first was that the dental chair is installed in a patient care space, defined in Article 100 of the National Electrical Code, so the requirements in Part II of Article 517 are triggered. 

This means that two equipment grounding conductor (EGC) paths are required for the branch circuits serving the patient care space. Because the contractor had installed PVC conduit in the underground to the chair location, the installation was already in violation of Section 517.13, which is more restrictive than the general requirements in NEC chapters 1–4.

Article 517

Article 517 provides specific definitions and rules that assist in identifying patient care spaces and determining where more restrictive grounding and bonding is required.

Typically, the engineering design and correspondence with the healthcare facility’s governing body will have already determined what are patient care spaces, and the design team will have specified suitable wiring methods that meet the redundant equipment grounding requirements.

Branch circuits in patient care spaces are required to provide two independent equipment grounding paths for all non-­current-carrying conductive surfaces of fixed electrical equipment likely to become energized and subject to personnel contact. 

Section 517.13(A) includes requirements related to the type of wiring method that can be used for branch circuit conductors. Approved wiring methods for patient care spaces include metal raceway systems or cables with a metallic armor or sheath acceptable as an EGC in accordance with 250.118. This is the first problem, in that the PVC conduit in the slab to the dental chair did not meet the requirements in 517.13(A). 

Since the concrete was already poured, the option to cut it and install a metallic conduit in the slab to meet the 517.13(A) requirement was extensive. The contractor apparently decided to use a workaround and install a flexible cable wiring method in the existing PVC, which would act as a sleeve.

Section 517.13 recognizes that metallic cable wiring methods could be used as long as the metal sheath of the assembly qualifies as an EGC, and the contained EGC is copper and insulated. It is important to recognize the types of conduit, tubing, cables and other wiring methods acceptable as EGCs in accordance with 250.118. A review of this section reveals a long list of qualifying wiring methods, including RMC, IMC, EMT, armored cable (Type AC) and metal-clad cable (Type MC), among others. 

These wiring methods inherently provide a path for ground-fault current through the raceway or metallic cable armor itself. The wiring methods listed in 250.118 that meet the minimum requirements as permitted EGCs are acceptable as wiring methods for branch circuits in patient care spaces.

Wet-location-rated conduit

The original violation cited only mentioned Section 517.13 requirements. Type AC cable appeared to meet the redundant equipment grounding requirements in 517.13. While it seemed an effective solution had been applied, there were other identified compliance issues that needed to be addressed.

The inside of the underground PVC conduit is considered a wet location as defined in Article 100. Section 300.5(B) indicates that the inside of conduit installed underground is considered a wet location. Section 320.12 restricts Type AC cable from being installed in a damp or wet location. This first workaround, while well-intended, did not meet the applicable minimum wiring requirements in chapters 1–4 and Section 517.13.

The contractor was faced with either saw-cutting the floor and installing metal conduit or wiring that meets the qualifying requirements in 250.118, or finding another alternative metallic cabling method that provides the two EGC paths addressed in 517.13 and is suitable for installation in a wet location. As a result, the contractor chose a suitable metal-clad cable identified for use in a wet location instead of saw-cutting the slab for the underground branch circuits supplying the dental chairs.

Sometimes, meeting and communicating with the AHJ can result in a mutual understanding of how to achieve Code compliance. No one likes to do things over, and communication can certainly help keep these types of problems from occurring. /

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].






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